Real-time x-ray tracking of moving targets, for example implanted tracking markers, is known in principle in the context of radiotherapy and/or radiosurgery. Thus, for example, U.S. Pat. No. 5,207,223 describes a method and apparatus for selectively irradiating a target within a patient, wherein images of the target area and of a marker implanted in its vicinity are repeatedly taken using two x-ray image recording systems arranged at an angle to each other. The data obtained can be used to establish, in real time, where the marker and therefore the target area is actually situated at each point in time. This is particularly important for targets or target areas which move (for example movement of the target area with movement of the patient due to the patient's breathing). In this method, both x-ray image detecting units are actuated at the same time or substantially the same time. As a result, the two viewing lines for a moving object intersect in three-dimensional space at the same time and can be used to determine the detected position of the marker and/or target area for a particular point in time.
Given the high scanning rates used, which are necessary to precisely track the movement, such systems expose the patient to a relatively high radiation load when both x-ray systems are respectively activated at the same time.